fix(audio_mixer): drop dangling speech_dup pad in full_mix ducking

full_mix with ducking enabled (the default) failed for a single narration
track + one music bed — the most common shape — because the ducking branch
appended an acopy[speech_dup] filter whose output pad was never consumed,
leaving the filtergraph with a dangling output that ffmpeg rejects.

For a single speech track speech_out is '[a0]' (starts with '[a'), so the
guarded append fired; the compensating pop() only removes the empty-string
case from the multi-speech branch, so the dead pad survived exactly in the
single-narration case. The speech stream is already re-derived for the final
mix via [speech_out], and ffmpeg auto-splits the reused input label, so the
duplicate is unnecessary. Multi-speech and SFX paths are unaffected.

Adds regression tests for single- and multi-narration full_mix with ducking.

Closes #265

tests/tools/test_audio_mixer_ducking.py

@@ -0,0 +1,89 @@
+"""Regression tests for audio_mixer full_mix ducking filtergraph.
+
+The ducking branch built an `acopy[speech_dup]` filter whose output pad was
+never consumed, leaving the FFmpeg filtergraph with a dangling output. FFmpeg
+rejects that, so `full_mix` with the most common shape — a single narration
+track plus one music bed, with ducking enabled (the default) — always failed.
+"""
+
+import shutil
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+from tools.audio.audio_mixer import AudioMixer  # noqa: E402
+
+pytestmark = pytest.mark.skipif(
+    shutil.which("ffmpeg") is None, reason="ffmpeg required for full_mix"
+)
+
+
+def _sine(path: Path, freq: int, dur: int) -> None:
+    subprocess.run(
+        ["ffmpeg", "-y", "-f", "lavfi", "-i", f"sine=frequency={freq}:duration={dur}", str(path)],
+        capture_output=True,
+        check=True,
+        timeout=30,
+    )
+
+
+def _has_audio(path: Path) -> bool:
+    out = subprocess.run(
+        ["ffprobe", "-v", "error", "-select_streams", "a",
+         "-show_entries", "stream=codec_type", "-of", "csv=p=0", str(path)],
+        capture_output=True, text=True, timeout=30,
+    )
+    return "audio" in out.stdout
+
+
+def test_full_mix_single_narration_plus_music_with_ducking(tmp_path):
+    speech = tmp_path / "speech.wav"
+    music = tmp_path / "music.wav"
+    _sine(speech, 440, 2)
+    _sine(music, 220, 3)
+    out = tmp_path / "mixed.wav"
+
+    result = AudioMixer().execute(
+        {
+            "operation": "full_mix",
+            "tracks": [
+                {"path": str(speech), "role": "speech"},
+                {"path": str(music), "role": "music"},
+            ],
+            "ducking": {"enabled": True},
+            "output_path": str(out),
+        }
+    )
+
+    assert result.success is True, result.error
+    assert out.exists() and _has_audio(out)
+
+
+def test_full_mix_multi_narration_plus_music_with_ducking(tmp_path):
+    s1, s2 = tmp_path / "s1.wav", tmp_path / "s2.wav"
+    music = tmp_path / "music.wav"
+    _sine(s1, 440, 2)
+    _sine(s2, 330, 2)
+    _sine(music, 220, 3)
+    out = tmp_path / "mixed_multi.wav"
+
+    result = AudioMixer().execute(
+        {
+            "operation": "full_mix",
+            "tracks": [
+                {"path": str(s1), "role": "speech"},
+                {"path": str(s2), "role": "speech"},
+                {"path": str(music), "role": "music"},
+            ],
+            "ducking": {"enabled": True},
+            "output_path": str(out),
+        }
+    )
+
+    assert result.success is True, result.error
+    assert out.exists() and _has_audio(out)